Reluctantly, he presses a combination of numbers into the keypad next to the gate. It opens, then closes behind us after we’ve driven in. The drive sweeps around in front of the house to what looks like a parking area, where I pull up beside Adam’s car and get out.

‘What’s it worth?’ I stare up at the impressive elevations. The house is old and gracious, too much so for someone like Mason. Its stone exterior has also been immaculately restored.

‘It’s hard to say until I’ve been inside.’ Clearly not pleased that he’s had to let us in, Matterson is brusque. ‘I’d imagine somewhere between five and six million. Now, if you’ll excuse me, I have to get on.’

Inside, the house is no less vast, its grandeur understated, its history evident in mullioned windows, arched doorways, stone floors worn smooth from age. While Adam walks from room to room recording measurements, Sarah and I start our own search.

As we walk into yet another beautiful room, she shakes her head. ‘Some house, isn’t it?’

I nod, not wanting to dwell on where Mason’s money may have come from. I start carefully looking through a chest of drawers, finding nothing personal. We search more drawers and cupboards in each room but everywhere we look, it’s the same. Everything is neat and orderly and sparse, as though Philip Mason always expected the police to come here.

When Sarah finds his office, my hopes rise that at last we’ll discover something. ‘What an office.’

A large desk is positioned by the window, with breath-taking views across the landscaped gardens towards the coast. There’s a state-of-the-art screen and keyboard, a pair of speakers positioned discreetly and heavy curtains hanging in the windows, but that’s all.

When a search of the desk reveals nothing, Sarah looks at me. ‘Frustrating, isn’t it? You’d have thought there’d be something.’

I shake my head. ‘There wouldn’t be, would there? Not if he was prepared to let an unescorted estate agent wander around. I wouldn’t mind betting the whole house is completely clean. But we still need to look. Keep your eyes out for letters and photos. Anything that might tell us where he is.’

I’m hoping that we’ll stumble upon wherever Mason’s hidden his personal effects. When our search turns up nothing of interest, we wait in the kitchen while Adam finishes up measuring the outbuildings. When he comes back in, he looks perplexed.

‘Is something wrong?’

He shakes his head. ‘It’s probably nothing. It’s just that some of the measurements don’t add up. I was redoing them to check I hadn’t made a mistake.’

I’m suddenly very interested in seeing Adam’s notes. ‘Where is this?’

He’s visibly less hostile than when we arrived. ‘Outside. I’ll show you.’